There are a few reasons why you might not be able to use the brush tool in Illustrator. One reason is that Illustrator does not support brushes in the same way as other Adobe Photoshop tools. For example, when you use the brush tool to draw a line, Illustrator will create a path that is based on the data that you provide. This means that if you try to use the brush tool to make a circle or an ellipse, your line will be too wide or too short, respectively.
